Your smart scale is probably not precise enough to be useful

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Photo: Koldunov Alexey (Shutterstock)

Your weight on the scale changes for so many reasons. We can gain or lose muscle, and we can gain or lose fat. We may be lighter when we are dehydrated, or heavier because we ate a lot of salt last night and are retaining water. Wouldn’t it be great to differentiate between all of these things? This is what many smart scales promise.

How scales measure your body composition

The weight is quite simple: YYou step on the scale, and it measures the force with which gravity pulls on your body.

Body fat, muscle mass, bone mass, body water, and other measurements (which we can summarize as “body composition”) come from something called bioimpedance analysis, or BIA.

Conductive pads on your scale (which can be superimposed almost invisibly in the material of the scale) pass a current through your body, from one foot to the other. You will not feel this current and it is not dangerous, although if you have a pacemaker you may be told that it is best to turn this feature off.

The scale can sense the electrical resistance between one foot and the other, which means that the current rises on one leg, passes through your torso, and descends the other leg. In essence, the formula imagines your legs and hips as a sort of horseshoe shape made from a mixture of water and non-water. It uses your height (which you enter into the app when you set up a profile) to guess the length of this horseshoe shape. And based on the resistance it measures, it comes up with an estimate of how much fatty tissue in your body is compared to aqueous tissue.

As you would expect, there are numerous limits to this approach. Most important is the number of assumptions the formula has to make on your body to go from “this is a 5’6” human with a foot-to-foot resistance value of X “to” your body is at 32%. fat ‘, much less to estimate the amount of bone or lean muscle tissue.

For example, it makes assumptions about the amount of fat, muscle, etc., in your upper body compared to your lower body. It can also be affected by your hydration level, when you exercised, and whether you have artificial knees or hips.

A review the article on the accuracy of the BIA points out that, because it takes so many assumptions, even the best devices and algorithms will probably never be able to provide fully accurate and detailed information about an individual’s body composition:

A fundamental problem is that BIA is a predictive method that inherently requires simplifications and assumptions based on population mean values, but considered to be applicable with precision to all subjects.

A 2016 Consumer Reports study found that the scales tested differed 21% to 34% from a BodPod measurement of body fat. It’s not really specific enough to know for sure what your body fat percentage is. If you really want a number, any number, a simple calculation like the Marine body fat estimator will probably be just as good.

Some people say they like to see their body fat (or muscle mass) change over time, and that it goes in the direction they expect. Your experience may vary on this. My Withings smart scale has never shown a significant change in these numbers to me, even when I know my body composition has changed. (And yes, I tried to turn on “athlete” mode. Even though I have more muscle than the average person my height, the numbers are getting even. less realistic for me with athlete mode.)

What about all the other numbers?

A large part of the market for smart scales is the number of features they have. A scale that can tell you 12 things sounds more sophisticated than one that can only tell you one or two measurements, doesn’t it?

If you enter your height in the app and assuming the scale is accurate enough to determine your weight, your measurements BMI should be correct, because BMI is just the relationship between your height and your weight.

Some scales claim to measure your BMR (basal metabolic rate), but it’s usually calculated from your weight, height, and age. You can get it from an internet calculator.

Measurements of muscle, fat, bone and body water are generally only adjustments to the body fat formula calculated by BIA; for example, lean body mass is your total weight minus body fat. These measurements all have the same accuracy issues that we discussed above about BIA. I wouldn’t put a lot of stock in any of them.

Some scales measure heart rate, which you can check yourself; Just count your pulse beats while looking at a clock and see if that matches the number on your scale.

Take a close look at the other numbers your scale claims to report, and you’ll likely see that they’re calculated from the data you’ve entered into your profile in the app, often in conjunction with your weight or other scale data.

So, are these scales precise enough to be useful? IIn my opinion, no. If you find the readings consistent enough to be tracked over time, go ahead and have fun with them. But I wouldn’t use the numbers on a smart scale (except for weight) to make decisions about what to eat or how to train.


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